Metallocene-catalyzed polymerization processes are well known in the art. Such processes employ catalyst systems which utilize metallocene compounds for the polymerization of olefinically unsaturated olefins. Metallocene compounds are defined as organometallic coordination compounds obtained as cyclopentadienyl derivatives of a transition metal. Processes which employ multiple metallocenes in a single polymerization reactor are also known. Bridged and unbridged biscyclopentadienyl Group 4 metal compounds are particularly representative; many are said to be useful for gas-phase polymerization or slurry polymerization where the use of supported catalysts is typical. For example, U.S. Pat. No. 4,808,561 describes a process for the polymerization of ethylene and other olefins, and particularly homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins and/or diolefins and/or cyclic olefins in the presence of a metallocene catalyst.
European Patent Application 0 659 773 describes a gas phase process for producing polyethylene employing at least one bridged metallocene catalyst, and optionally, one or more second catalysts which may be non-bridged metallocene catalysts. In Examples 6.sup.c and 7.sup.c diphenylmethylene(cyclopentadienyl)(fluorenyl) zirconium dichloride was utilized to produce polymers having densities of 0.928 and 0.920, respectively, and MFR's of 47 and 37, respectively.
U.S. Pat. No. 5,324,801 describes a process for the preparation of a cycloolefin copolymer using specific metallocenes having mononuclear or polynuclear hydrocarbon radicals which are able to form a sandwich compound with the central metal atom. The mononuclear or polynuclear hydrocarbon radicals are linked by a single- or multi-membered bridge.
European Patent Application 0 619 325 describes the preparation of polyolefins having a multimodal or at least bimodal molecular weight distribution using a catalyst system comprising, inter alia, at least two metallocenes containing the same transition metal and selected from mono, di, and tri-cyclopentadienyls and substituted cyclopentadienyls of a transition metal wherein at least one of the metallocenes is bridged and at least one is unbridged. Preferably, the unbridged metallocene is a bis(cyclopentadienyl) zirconium dichloride. In Table 1, ethylene polymers having HLMI/MI.sub.2 ratios greater than 38 and densities ranging from 0.9408 and 0.9521 are reported. Activities ranging from 662 to 1126 g/g.h are reported.
U.S. Pat. No. 5,405,922 describes a gas phase polymerization process for polymerizing olefins utilizing a metallocene in a gas phase fluidized bed polymerization reactor operating in a condensed mode. In Tables 1-4, ethylene polymers having densities ranging from 0.9168 to 0.9222 g/cc are reported.
PCT publication WO 95/12622 reports in Examples 1-7 and Table 1 polymerization results for catalysts employing bis(cyclopentadienyl) zirconium dichloride or bis(indenyl) zirconium dichloride.
While metallocene-catalyzed ethylene polymerization processes are well known in the art, certain problems with these processes remain. Metallocenes, compared to transition metal halide polymerization catalysts, are expensive materials. If the metallocene catalyst productivity is too low, the process will not be economical. This problem is aggravated when metallocene-catalyzed processes are used to make higher density ethylene polymers, such as medium density ethylene/.alpha.-olefin copolymers (MDPE), or high density ethylene/.alpha.-olefin copolymers and homopolymers (HDPE), because metallocene catalysts, like other catalysts, generally exhibit lower catalyst productivity under HPDE or MDPE polymerization conditions in comparison with low density conditions. The magnitude of the activity loss under MDPE or HDPE conditions is even more severe for metallocene catalysts compared to conventional Ziegler-Natta catalysts.
Moreover, low productivity metallocene processes, like other low productivity processes, may suffer from poor operability. In particle-form polymerization processes, such as gas phase and slurry processes, lower catalyst productivity generally results in reduced average particle size (APS) and higher fines levels. Fines are readily carried over into the cycle gas loop of a fluidized-bed gas phase reactor, where can they can foul the cycle gas cooler and the reactor distributor plate, thereby inhibiting effective reactor cooling and bed fluidization. If the fines level becomes excessive, the reactor may become inoperable and require a shut-down and cleaning, resulting in lost production and increased costs.
High fines levels are especially a problem when gas phase processes are used to make HDPE. HDPE, compared to lower density polymers, will generally have more fines, even if the catalyst productivity is comparable to the catalyst productivity in a lower density process. The problem is aggravated even further if the catalyst used to make the HDPE exhibits low productivity under HDPE conditions. Thus, there is still a need for a metallocene-catalyzed ethylene polymerization process that utilizes a simple, inexpensive metallocene and operates with a higher catalyst productivity, especially during the production of HDPE.
In addition, when slurry and gas phase processes are used to make ethylene/.alpha.-olefin copolymers of a given density, it is desirable to use a catalyst that exhibits superior a-olefin incorporation. Catalysts exhibiting superior .alpha.-olefin incorporation require that, for a given reactor concentration of ethylene, less .alpha.-olefin need be present in the process to achieve a given polymer density. For example, a highly incorporating catalyst can produce a low density polyethylene with a low ratio of .alpha.-olefin/ethylene reactants. This is advantageous because a higher concentration of .alpha.-olefin produces a higher concentration of dissolved .alpha.-olefin in the polymer particles, rendering the particles sticky and prone to agglomeration, chunk formation, and fouling. The problem becomes especially acute when polymers having densities below about 0.915 g/cc are produced. Further, for gas phase processes operating in condensed mode, as described for example in U.S. Pat. No. 5,462,999 and U.S. Pat. No. 5,405,922, it is especially desirable to minimize the concentration of .alpha.-olefin necessary to achieve a given polymer density. The ability to use less .alpha.-olefin permits higher levels of condensed liquid to be employed in condensed mode operation, which in turn permits higher production rates. Thus, there is still a need for a metallocene-catalyzed ethylene polymerization process that produces polymers of a given density using the lowest possible level of .alpha.-olefin comonomer.
Finally, it would be highly desirable to have a metallocene-catalyzed ethylene polymerization process that meets the above needs and at the same time provides products having the expected beneficial characteristics of metallocene-catalyzed products, especially narrow molecular weight distribution as indicated by a low ratio of HLMI/MI.